The present invention relates to an air-fuel ratio control system for an internal combustion engine, which system controls the air-fuel mixture to the stoichiometric air-fuel ratio at which ratio a three-way catalyst acts most effectively.
In a known air-fuel ratio control system for a motor vehicle, the air-fuel ratio of the air-fuel mixture burned in the engine cylinders is detected as the oxygen concentration in the exhaust gases by means of an O.sub.2 sensor provided in the exhaust system of the engine, and a decision is made dependent on the output signal from the O.sub.2 sensor which indicates whether the air-fuel ratio is richer or leaner than the value corresponding to the stoichiometric air-fuel ratio for producing a control signal. The control signal is applied to a proportion and integration circuit (PI circuit), the output of which is changed to pulse form. The pulses operate an electromagnetic valve so as to control the amount of bleed air in a carburetor for controlling the air-fuel ratio of the mixture. When the duty ratio of the pulses is reduced, the air-fuel mixture is enriched. Thus, the air-fuel ratio is controlled to the stoichiometric air-fuel ratio at which a three-way catalyst in the exhaust system acts most effectively. In such an air-fuel ratio control system, when the vehicle is accelerated or decelerated, the air-fuel ratio is liable to deviate from the stoichiometric air-fuel ratio.
In order to rapidly converge the deviated air-fuel ratio to the stoichiometric air-fuel ratio, the constant of the PI circuit is changed to a large value. The constant of the PI circuit is stepwisely changed to several values in accordance with driving conditions of the vehicle. The constant of the PI circuit is decreased to a small value at engine idling operation, because the air-fuel ratio does not vary largely at idling.
As shown in FIG. 5(b), when the vehicle is accelerated, the air-fuel mixture is enriched by the operation of the carburetor of the engine, in order to meet the requirement of the acceleration. On the other hand, the air-fuel ratio control system operates to dilute the air-fuel mixture, which is performed by increasing the duty ratio, at a large constant of the PI circuit, as shown in FIG. 5(a). When an accelerator pedal is released to decelerate the vehicle, the duty ratio decreases as shown by a solid line in FIG. 5(a) at a constant of the PI circuit. In a conventional system, when the accelerator pedal is released to idle the engine, the constant of the PI circuit becomes small as described above. Accordingly, the duty ratio slowly decreases, and hence the mixture is slowly enriched as shown in FIG. 5(b). As a result, the mixture remains lean at idling operation, which renders the engine idling operation unstable.